Recent Advances in Functional Transparent Semiconductor Films and Coatings

The contributions to this Special Issue, Recent Advances in Functional Transparent Semiconductor Films and Coatings, demonstrate that these materials are now essential to optoelectronic, photocatalytic, and energy detection and conversion technology. They demonstrate how the interaction between fundamental physics and practical device integration is driving rapid progress in materials chemistry, thin-film deposition, nanoscale characterisation, and device engineering. The contributions gathered under this theme offer a compelling overview of current research dynamics, with each one highlighting innovative strategies for adjusting the structure, optical response, functionality, and engineering of semiconductor-based devices, as well as the development of more durable protective coatings. Emerging synthesis techniques, ranging from low-temperature atomic layer deposition to solution-processed nanostructures, are broadening the technological scope of transparent semiconductors to include flexible substrate deposition, large-scale manufacturing, and environmentally friendly production methods.

Zinc oxide (ZnO) stands out in particular as a remarkably multivalent material. ZnO thin films with multicomponent composite structures constitute a promising development direction in the domain of transparent conductive oxide (TCO) thin films and coatings [1,2,3,4]. In their study, Abou Zeid et al. investigated the antifungal, antibacterial, and photocatalytic properties of ZnO nanostructures [5]. They synthesised homogeneous, well-ordered ZnO nanowires (NWs) using an environmentally friendly, economical, bottom-up hydrothermal technique. Their vertically aligned nanowire arrays exhibit the complete degradation of methylene blue (MB) under UV illumination, as well as the severe inhibition of Pseudomonas putida growth, through a combination of physical contact, the release of Zn²⁺ ions, and the generation of reactive oxygen species (ROS). The morphology of these NWs, combined with Zn²⁺ ion release, disrupts the bacterial cell membrane, thus conferring antibacterial properties in the dark that are enhanced under UV illumination. Additionally, ZnO NWs exhibit excellent photocatalytic properties under UV light, enhancing their antibacterial effects. This dual-function behaviour, which is mediated mechanically and chemically in the dark and catalysed by light, demonstrates how the nanometric morphology of transparent semiconductors can be exploited to address global challenges relating to hygiene and the environment.

In the same field, Khiari et al. developed high-performance ZnO/Ag nanocomposite thin films using a single-step deposition process that combines sol–gel and spin-coating techniques [6]. This work is notable for the direct incorporation of Ag nanoparticles (NPs) into the ZnO precursor solution and their uniform ultrasonic dispersion. The presence of Ag NPs enhances the photocatalytic activity of the nanocomposite film when exposed to both UV light and natural sunlight, thereby overcoming the traditional spectral limitations of ZnO due to its wide bandgap. The authors identified an optimal Ag NP content. They demonstrated the stability and recyclability of their photocatalysts over several laboratory and natural environment cycles for the degradation of indigo carmine, thus highlighting the practical viability of their ZnO-based nanocomposite thin films for real-life water treatment applications. This study further emphasises the scientific and technological significance of incorporating plasmonic NPs into transparent conductive oxide matrices such as ZnO.

Stefanov explored the properties of thin films made from two similar transparent conducting oxides: ZnO and TiO₂ [7]. These films were produced using a solution containing salicylate as the chelating agent for both oxides and were deposited by dip-coating. Controlling the precursor solution enables the simultaneous adjustment of optical transparency (between 70% and 90%), film thickness and reactive surface area for photocatalysis. ZnO films exhibited greater photocatalytic efficiency than TiO₂ in the photodegradation of methylene blue (MB). However, the author highlights the adverse effect of prolonged exposure to UV light in the case of ZnO. They attribute this to the possible photodissolution of ZnO. This work paves the way for further research into optimising film properties and addressing the challenges associated with photodissolution. Such nuanced knowledge is essential for developing transparent coatings intended for long-term outdoor use or in harsh environments. The films obtained in this study are transparent and have good photocatalytic properties, making them promising candidates for use in transparent coatings for environmental remediation and in advanced photoelectrochemical and photonic devices.

Vuillermet et al. investigated the impact of implanting n-type and p-type dopants (nitrogen and aluminium, respectively) into silicon carbide (SiC), which is another wide-bandgap semiconductor that is optically transparent and essential for technological applications beyond photocatalysis [8]. Using cathodoluminescence at 80 K, they mapped the creation, evolution, and optical signatures of silicon vacancies and those generated by controlled ion implantation and annealing, in order to better understand their role in nanophotonics and quantum technologies. Their results demonstrate that ion implantation and annealing can be incorporated into the production of nanophotonic devices in 4H-SiC. These well-established industrial processes can be adapted to incorporate optically active defect centres into transparent semiconductor platforms, paving the way for increasingly sophisticated device architectures.

Also working at the nanometric scale, Benaouin et al. studied the thickness and composition of self-healing oxide coatings that form on iron- and nickel-based alloys when they are subjected to thermal oxidation at different oxygen pressures [9]. They exploited the sensitivity of UV–visible ellipsometry as a surface probe to study the surface area modified by oxygen. These measurements, which were confirmed by glow discharge optical emission spectrometry (GDOES), revealed that both alloys consist of two layers in this zone: an Fe-rich layer on top of a Cr-rich layer in the case of AISI 304 stainless steel and a NiO-type layer on top of a NiCr-type layer in the case of Inconel 600. This study reveals subtle changes in the composition and density of these films. This knowledge could benefit industries operating in oxidation-sensitive environments, ranging from nuclear energy systems to high-temperature engineering. Their demonstration of ellipsometry as a powerful, non-destructive tool for characterising absorbing oxide bilayers could also lead to its wider adoption in monitoring transparent and semi-transparent protective coatings.

These contributions are united by the exploration of the remarkable properties of transparent semiconductor oxide-based coatings, which are produced using a variety of techniques. The aim of these fabrication techniques is to control the properties of the coatings without compromising their functionality. Consequently, these materials exhibit remarkable properties alongside antibacterial activity, photocatalytic efficiency, defect-mediated photonics and corrosion resistance. The conclusions, perspectives and discussions presented in this issue are intended to inspire further research, spark innovative ideas and motivate new applications that will push the boundaries of what transparent materials can achieve. Despite industry concerns about the long-term durability of products based on conductive transparent oxide coatings, the market for these films and coatings is growing rapidly [10] due to the increasing demand for touchscreens, flexible and intelligent electronics [11,12,13,14], and the expansion of the solar energy sector [15].

We would like to express our gratitude to the authors who contributed to this Special Issue, as well as to the reviewers who helped to ensure the quality of its publications.